A steering mechanisms known as an Ackerman-Jeantaud steering mechanism includes a steering link mechanism connecting together right and left vehicle wheels (an assembly including a tire, a rim, a hub, an in-wheel motor, etc.) is hereinafter referred to as “vehicle wheel” or simply “wheel”, and used to steer the vehicle wheels. This steering mechanism includes tie rods and knuckle arms such that while the vehicle is turning, the right and left wheels move around the same center point.
There is also known a steering mechanism including an actuator capable of changing the lengths of the tie rods, the distance between the (right and left) tie rods, or the angles between the respective wheels and the knuckle arms. This steering mechanism allows smooth normal traveling, smooth parallel traveling, and smooth small-radius turning, and is sufficiently responsive (see, for example, JP Patent Publication 04-262971A).
There are also known steering mechanisms disposed, respectively, between the front right and front left wheels and between the rear right and rear left wheels, and each including right and left separate rack bars rotatable about their axes, and a forward-and-reverse switching means provided between the rack bars. The forward-and-reverse switching means is capable of transmitting the rotation of one of the rack bars to the other rack bar so that the other rack bar is rotated selectively in one and the other direction. This steering device allows the wheels to form a steering angle of 90 degrees, and allows lateral movement of the vehicle (see, for example, JP Patent Publication 2007-22159A).
A four-wheel steering vehicle is also known including an actuator actuated when the front wheels are steered to steer the rear wheels (see, for example, JP Utility Model Registration 2600374). Another steering mechanism includes a rack housing connecting right and left wheels together, and movable in the fore-and-aft direction to perform toe adjustment of the right and left wheels, thereby improving travel stability (see, for example, JP Patent Publication 2003-127876A).
A steering mechanism is also known which includes two rack bars movable to the right and left independently of each other, and connected, respectively, to the right and left wheels through tie rods, such that the rack bars are movable in the opposite directions to each other relative to a synchronizing gear box by means of synchronizing gears retained in the synchronizing gear box (see JP Patent Application 2013-158876).
An ordinary Ackerman-Jeantaud steering mechanism allows smooth normal travel because, during normal traveling, the lines perpendicular, in plan view, to the lines of rotation of the respective wheels (widthwise center lines of the wheels) converge on the center of turning of the vehicle. However, even if it is desired to move the vehicle laterally (parallel movement in the lateral direction with respect to the fore-and-aft direction of the vehicle), it is difficult to steer the wheels in the direction perpendicular to the fore-and-aft direction of the vehicle due to the lengths of the steering links and interference with other members. Also, even if one of the right and left wheels is steered by 90 degrees, the one and the other of the right and left wheels never becomes completely parallel to each other, thus making smooth travel difficult.
In these type of vehicles, the front wheels as the main steered wheels are steerable in a predetermined travel direction, while the rear wheels as the auxiliary steered wheels are arranged parallel to the fore-and-aft direction of the vehicle. Thus, when the vehicle is turned by steering the front wheels, the rear wheels follow different turning circles from those of the front wheels. In particular, while the vehicle is traveling at a low speed, the rear wheels follow circles located inside of the circles followed by the respective front wheels, and while the vehicle is traveling at a high speed, under a centrifugal force, the front wheels follow circles located inside of the circles followed by the respective rear wheels. That is, when the front wheels are steered in the direction in which the vehicle is supposed to be turned, the vehicle cannot be turned in this direction. A vehicle including a four-wheel steering mechanism, which steers not only the front wheels but also the rear wheels, is free of this problem and improves travel stability.
The vehicle with a four-wheel steering mechanism (known as “4WS vehicle”) disclosed in JP Patent Publication 04-262971A allows lateral traveling and small-radius turning, of the vehicle. However, since this vehicle include a large number of actuators for changing the lengths of the tie rods, the distance between the (right and left) tie rods, and the angles between the respective wheels and the knuckle arms, control of the actuators is complicated. The mechanism disclosed in JP Patent Publication 2007-22159A is not only complicated in structure, but also a large number of gears are used to steer the wheels by rotating the rack bars, which tends to lead to looseness and makes smooth steering of the wheels difficult.
JP Utility Model Registration 2600374 discloses a conventional four-wheel steering mechanism. While this mechanism allows steering of the rear wheels, but does not enable lateral traveling for the same reasons as set forth above. The mechanism disclosed in JP Patent Publication 2003-127876A enables toe adjustment, but does not allow lateral traveling and small-radius turning.
JP Patent Application 2013-158876 is a patent application filed by the present applicant, solves the above-mentioned problems of the other JP patent documents discussed above, and is capable of performing basic four-wheel steering functions without a problem. However, while the vehicle is traveling on a snow-covered road, or otherwise, the road condition is not favorable, if the frictional states of the road surfaces with which the right and left wheels are in contact are extremely different from each other, since the rack bars are connected to the right and left wheels, respectively, depending on e.g. the difference in frictional states between the road surfaces with which the right and left wheels are in contact, when the wheels are steered, one of the rack bars may stop, or the right and left wheels are steered at different steering speeds. This makes it impossible to symmetrically steer the right and left wheels, and thus to eventually achieve the target wheel angle. For example, if the frictional force between the left wheel and the road surface is larger than the frictional force between the right wheel and the road surface, only the right wheel could be steered markedly with the left wheel not steered. (This is because the rack bar connected to the left wheel does not move, so that the rack bar connected to the right wheel moves a distance which is the sum of the movements of the right and left rack bars while the vehicle is traveling normally.)